J . Phys. Chem. 1987, 91, 5279-5286
5279
Dynamics, Order, and Molecular Conformation of Micellar Zwitterionic (Decyldimethy1ammonio)propanesulfonate Mikael Jansson,* Puyong Li, and Peter Stilbst Institute of Physical Chemistry, Uppsala University, S - 751 21 Uppsala, Sweden (Received: February 2, 1987)
The so-called two-step model has been applied to multifield 13C relaxation and NOE data to determine order parameters and correlation times of (decyldimethy1ammonio)propanesulfonate(DAPS) in pure and octanol-swollen DAPS micelles. Additional conformational information was obtained from 13C shift measurements above and below the critical micellization concentration. The combined results indicate that the zwitterionic headgroup is motionally restricted as compared to micellized ionic surfactant headgroups and that it is tilted relative to the normal of the micelle surface. Self-diffusion measurements demonstrate that micellar inclusion of octanol has large stabilizing effects of the DAPS micelle, decreasing the nonmicellized DAPS concentration dramatically. However, in the I3C relaxation and NOE data no significant differences in order parameters and correlation times of DAPS are detected in the presence of octanol. 13C shift measurements demonstrate, nevertheless, that minor conformational changes of the micellized DAPS molecule are induced by octanol.
1. Introduction There is a growing interest in the dynamic nature of surfactant organization and average structural properties of surfactant systems. Nuclear magnetic spin relaxation is a powerful tool for studying these phenomena. The application of the so-called two-step model to 13C relaxation and 13CN O E data has been demonstrated to give relevant information on alkyl chain order and correlation times corresponding to different dynamic modes in the surfactant Conformational information of the surfactant can in addition be obtained from I3C shift measurements, which provide semiquantitative data on gauche and trans conformational populations in the alkyl chain.6s In the present communication we have used I3Crelaxation, NOE, and chemical shift data, together with self-diffusion data which monitor the size of the micelles and the free amphiphile concentration, to study the micellized zwitterionic surfactant (decyldimethy1ammonio)propanesulfonate (DAPS). The effect of micellar inclusion of octanol on the conformation and orientation of DAPS was also examined. Due to the difference in headgroup size and alkyl chain length of DAPS and octanol, micellarly incorporated octanol can be anticipated to change the packing of both the headgroups and the alkyl chains, which would be reflected in the dynamics and conformation of the surfactant. 2. Materials
(Decyldimethy1ammonio)propanesulfonateand (dodecyldimethy1ammonio)propanesulfonate (DAPS and DDAPS, respectively) were high-purity samples from Calbiochem, the alcohols (hexanol, octanol, and decanol) were obtained from Merck, and HMDS (hexamethyldisiloxane) and T M S (tetramethylsilane) were supplied from Stohler Isotope Chemicals. Deuterium oxide (99.8 wt %), which was used as a solvent, was purchased from Norsk Hydro, Rjukan, Norway. All samples were prepared by weighing directly in the N M R tubes. 3. Theory and Methods 3.1. Two-step Model. It is usually assumed that the relaxation of a I3C nucleus is dominantly governed by dipole-dipole interactions with its directly bonded protons. In this case, the general expressions for the NMR longitudinal relaxation rate R , and the nuclear Overhauser enhancement q (NOE) are given, in SI units, by9 Ri, = ( N I / 2 0 ) x (hyHTCh /4*rC-H3)2(J(wH- wC) + 3J(wC) + 6J(wH + w C ) ) (1) Department of Physical Chemistry, The Royal Institute of Technology, S-10044, Stockholm 70, Sweden.
0022-3654/87/2091-5279$01.50/0
91 = ( l /20)yH3yC(&h/4arC-H3)2(N~T1i)
+ wc) - J
(~J(WH
( ~ H Wc))
(2)
where the index i refers to the ith carbon along the alkyl chain, po represents the permeability of vacuum, yH and y c represent the magnetogyric ratios of the proton and the carbon nuclei, N, is the number of directly bonded protons to carbon i, h is Planck’s constant divided by 2a, rC-His the C-H bond length, and wH and wc denote angular Larmor frequencies of protons and carbons, respectively. The J(w)’s represent various reduced spectral densities at the indicated angular frequencies. Since the spectral density is the time-dependent part of the transition probability per unit time, as induced by fluctuating random fields, it contains interesting information at the molecular level. Molecular motion is commonly quantified in terms of an autocorrelation function of appropriate kind for the Brownian reorientation processes. The key problem with nuclear spin relaxation data is that one cannot directly determine the autocorrelation function from measured spin relaxation data; spin relaxation rates only provide information on the Fourier transform of the autocorrelation function at a very limited number of frequencies. For this reason one has to invoke a motional model for the reorientation processes to go from observed relaxation rates to parameters of the time-domain autocorrelation function for molecular reorientation. It is assumed in the so-called two-step model as applied to spherical that a fast, slightly anisotropic, local chain motion is superimposed on a slow isotropic overall motion of the micelle. If the autocorrelation functions of the fast and slow motions are single exponentials, the relevant reduced spectral densities will be given by J(W)
= (1 - S2)27cf-k 2 s 2 T c s / (1 -k
S = ( 1 / 2 ) ( 3 COS’ 8 - 1 )
(WTC~)~)
(3)
(4)
where f and s denote the correlation times of the fast and the slow (1) Wennerstrom, H.; Lindman, B.; Soderman, 0.;Drakenberg, H.; Rosenholm, J. B. J . Am. Chem. SOC.1979, 101, 6860. (2) Halle, B.; Wennerstrom, H . J . Chem. Phys. 1981, 75, 1928. (3) Walderhaug, H.; Soderman, 0.;Stilbs, P. J . Phys. Chem. 1984, 88,
1655. . ... (4) Siiderman, 0.; Walderhaug, H.; Henriksson, Chem. 1985, 89, 3693.
U.;Stilbs, P. J . Phys.
(5) Eriksson, P.; Khan, A.; Lindblom, G. J . Phys. Chem. 1982, 86, 387. (6) Grant, D. M.;Cheney, B. V. J . Am. Chem. SOC.1967, 89, 5315. (7) Cheney, B. V.;Grant, D. M. J . Am. Chem. SOC. 1967, 89, 5319. (8) Rosenholm, J. B.;Drakenberg, T.; Lindman, B. J . Colloid Interface Sci. 1978, 63, 538. (9) Doddrell, D.; Glushko, V.; Allerhand, A. J . Chem. Phys. 1972, 56, 3683.
0 1987 American Chemical Society
5280
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
motions, respectively, S is the order parameter, and 0 is the angle between the C-H vector and a local director perpendicular to the micellar surface. The molecular representation of the correlation times would thus be that T~~ characterizes the local motions of the C-H vector in the alkyl chain, e.g., rotation of the C-H bond due to gauche-trans isomerizations and torsions, whereas T~~ corresponds to the slower processes of aggregate tumbling and surfactant lateral diffusion at the micelle surface. It should be pointed out that for eq 3 to apply, the time scales of the fast and the slow motions must be sufficiently different in that the average over S is taken over a time long enough to provide a proper average of the fast local motion, but short enough not to include the slow motion. Furthermore, it is assumed that the fast local motion occurs is an environment which has, on the average, a threefold or higher symmetry where the symmetry axis is the local director. Under these conditions a single parameter (the order parameter, S) is needed to fully describe the orientation of each methylene group in the micellar frame.1-5 The variable field T , measurements of the present study were performed at three different magnetic fields, 7.076, 2.348, and 1.415 T, on a Varian XL-300 spectrometer, a JEOL FX-100 spectrometer, and a JEOL FX-60 spectrometer, respectively, all working in the FT mode. Nuclear Overhauser enhancements were determined on the Varian XL-300 spectrometer. When collecting T I data the inversion-recovery techniquelo was employed, using 16 T values and a pulse repetition time of 5Tl of the longest T I measured. The T I was extracted by fitting the peak intensities to the function Z ( T ) = Z0(1 - A exp(-T/T,)) (5) where Z ( T ) denotes the intensity at the time T , Zo is the equilibrium intensity, and A is a constant. The fitting procedure and confidence interval estimation, based on a nonlinear least-squares parameter optimization, were made with the Fortran UNIFIT computer program." When determining the NOES, the dynamic NOE technique'* was employed. Again 16 T values were used and the NOE was extracted by fitting the peak intensities to the function I ( T ) = 10(1 + II - 17 e x p ( - ~ / T ~ ) )
(6) where 17 denotes the NOE (at T = infinite) and Zo the peak intensity without NOE. Ti was predetermined by the more accurate inversion-recovery technique and was locked in the minimization. The observed R , values were corrected for the presence of minor amounts of nonassociated surfactant by assuming a two-site relaxation rate model R,(obsd) = pR,(mic) + (1 -p)Rl(free) (7) where p denotes the fraction of micellized surfactant (determined by self-diffusion measurements), R , (obsd) the observed relaxation rate, R,(mic) the micellar relaxation rate, and R,(free) the relaxation rate of the free surfactant, which was measured on a sample with a surfactant concentration below cmc. Analogous corrections were made for the N O E values. The individual fast correlation times, the order parameters, and the common slow correlation time were extracted by fitting the corrected Ti's and NOE's into eq 1 and 2, so that the residual square sum defined by error = x ( ( R l ( m i c ) - Rl(calcd))2/Rl(mic)2)+ X ( ( d m i c ) - v(calcd))2/dmic)2) (8) was minimized. Each observation is thus normalized to give equal weight to all observed data. The dipolar coupling constant used in eq 1 was calculated by assuming a carbon-proton bond length of 1.09 A, the value normally used for sp3 carbons. It has been demonstrated that uncertainties as to the magnitude of the interaction constant do have large effects on the results of the (10) Vold, R. L.; Waugh, J. S.; Klein. M. P.; Phelps, D. E. J . Chem. Phys. 1968, 48, 3831. (11) Stilbs, P.; Moseley, M. E. J . Magn. Reson. 1978, 31, 5 5 . (12) Freeman, R.; Hill, H. D. W.; Kaptein, R . J . Magn. Reson. 1972, 7 , 231.
Jansson et a]. twc-step model parameter eval~ation.'~However, an erroneously assumed dipolar coupling constant will not affect the relative magnitudes of S and T~~ at different positions along the carbon chain. Rl(calcd) and V(ca1cd) are the values calculated by eq 1 and 2, respectively, together with the spectral densities calculated by eq 3. The minimization and the confidence interval estimation were achieved by a specially written Fortran 77 computer program (Stilbs, P., unpublished; cf. ref 3 and 11). 3.2. Band-Shape Analysis. If the spectral densities of the two-step model can be extracted directly without invoking any model, the inherent assumptions of the model can be tested. A straightforward way is to carry out a band-shape analysis of the resonances of carbons directly bound to I4N. By using Redfield formalismI4 one can reformulate the standard band-shape exp r e s s i o n ~in~ ~terms of individual spectral densities. For a spin (I%) coupled to a spin 1 nucleus, the band shape is given Z(w) = -Re (l'.(A - i d - I . 1 )
(9)
where Z(o) is the absorption intensity of the spin (I3C) at frequency w, 1 a unit matrix of dimension 3, 1' the unit vector, and A the matrix given by A =
diag (iJC-N,O,-iJC-N)- ROI -J(wN)
[
-
2J(2wN)
+ (3a2/20)x2 X
J(Wh)
J(wN)
-2J(wv)
~ J ( ~ W N )
J(WN)
2J(20N)
J(4
- J ( w v ) - 2J(2Wh I
]
(10)
where JC-Ndenotes the scalar coupling constant (rad/s), wh the Larmor frequency of relaxing spin (I4N), a phenomenological transverse relaxation rate for the spin 1 / 2 (13C) accounting for all other relaxation contributions and magnetic field inhomogeneities. To extract spectral density parameters the 13C spectrum of D A B , measured at 75 MHz on the Varian XL-300 spectrometer, was digitally transferred to a VAX 11/780 computer. The parts of the spectrum (ca. 1000 points) containing the resonances of the methyl carbons directly bound to the I4N nuclei were subjected to a nonlinear least-squares fit to eq 10, using the procedure described earlier.20 Although some of the parameters are known and can be locked in the iteration (see section 4.25) this is in principle a seven-parameter optimization on the amplitude, the intensity of the base line, JC-N, Ro, a reference of the center of and R , for the relaxing the band shape, the ratio J(wN)/J(2wN), spin (I4N) that is described by2' Rl = ( 3 ~ ~ / 2 O ) x ' ( J ( + w )4 J ( 2 ~ ) )
(1 1 )
x denotes the quadrupolar coupling constant which in this case is not known but can be estimated in an indirect way. In the hexagonal phase of the surfactant x is related to the order parameter asZ2 x
= (8/3)(A/S)
(12)
where A is the measured peak separation in the Pake doublet of the I4N nucleus in the hexagonal phase. The hexagonal phase can be identified through the deuterium splitting pattern of Dz0.23 Sijderman, 0. J . Magn. Reson. 1986, 68, 296. Redfield, R. G. Adu. Magn. Reson. 1965, I , 1. Pople, J. R. Mol. Phys. 1958, I , 168. Martin, J. F.; Vold, R. L.; Vold, R. R. J . Magn. Reson. 1983,51, 164. Suzuki, M.; Kubo, R. Mol. Phys. 1963, 7, 201. Cuntliffe, A. V.; Harris, R. K. Mol. Phys. 1968, 15, 413. Witanowski, M.; Webb, G. R. Nitrogen N M R ; Plenum: New York, Stilbs, P.; Soderman, 0.;Walderhaug, H. J . Magn. Reson. 1986, 69, Abragram, A. The Principles of Nuclear Magnetism; Clarendon: U.K., 1961. Wennerstrom, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, (23) Khan, A,; Fontell, K.; Lindblom, G.; Lindman, B. J . Phys. Chem. 1982, 86, 4266.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5281
Conformation of (Decyldimethy1ammonio)propanesulfonate
2.6 2.4 2.2 2.0
l.a
-
j
em
gl
BB
1.6 1.4 1.2
1.0
0.8
-
a'
0
eum
I
I
l
,
,
I
I
I
-
l
-
-
-
l
-
!
l
l
l
l
3 . 6 3 . 4 3 . 2 3 . 0 2 . 8 2 . 6 2 . 4 2 . 2 2 . 0 1.8 1.6 1.4 1.2 1 . 0 0 . 8 0 . 6 PPM
Figure 1. Proton COSY spectra of 25 wt % ' (octyldimethylammonio)propanesulfonateat 300 MHz.
A was measured with a quadrupolar echo technique24on a JEOL FX-100 spectrometer, using a 67 wt % DDAPS sample in D 2 0 . 3.3. SelfDiffusion Measurements. The FT pulsed field gradient spin-echo (FT-PGSE) method25is a proven and powerful method for studying transport processes, applicable to a great number of different systemsz6 This method enables one to obtain the fraction of the micellar surfactant p , (see eq 7) and also to estimate the size of the micelles. Since there is a fast exchange between the surfactant in the aqueous phase and the micellar surfactants on the pertinent N M R time scale, the observed time-averaged self-diffusion coefficient D(obsd) will be described by D(obsd) = pD(mic) + (1 - p)D(free) (13) where D(mic) denotes the self-diffusion coefficient of the micelles and D(free) the self-diffusion coefficient of the nonmicellized surfactants. D(mic) becomes accessible by studying the diffusion rate of a hydrophobic micellar probe like, e.g., hexamethyldisiloxane (HMDS), whereas D(free) can be measured on a sample with a surfactant concentration below cmc. Such diffusion measurements were performed on a JEOL FX-100 spectrometer at 25.0 f 0.5 OC by monitoring the proton spin echoes by procedures described earlier.26-29 The signal amplitudes A ( 6 ) were subjected to a nonlinear fits according to A ( 6 ) = F exp(-27/T2 - D ( Y G S ) ~ ( A 6/3))
(14)
where F is a factor relating to the J-modulation effects (leading to positive or negative peaks), y the gyromagnetic ratio of protons, G the magnetic field gradient (which was calibrated against literature value of self-diffusion coefficient for trace amounts of (24) (25) (26) (27) (28) (29)
Davis, J . H. D. Biochim. Biophys. Acta 1983, 737, 117. Stejskal, E. 0.;Tanner, J. E. J . Chem. Phys. 1965, 42, 288. Stilbs, P. Prog. Nucl. Magn. Reson. Specfrosc. 1987, 19, 1. Jansson, M.; Stilbs, P. J . Phys. Chem. 1985, 89, 4868. Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 176. Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 215.
protons in heavy water), T2the transverse relaxation time, 6 the field gradient duration, A the time interval between the gradient pulse, and D the self-diffusion coefficient. 6 was varied in 16 steps between 20 and 90 ms while T and A were both kept constant at 140 ms. 3.4. 13CChemical Shift Measurements. Chemical shift effects of the micellized surfactant were interpreted by assuming a two-site exchange model on the exchange between micellized and free surfactant. In the fast exchange limit the observed chemical shift will be described by G(obsd) = p6(mic)
+ (1 - p)b(free)
(15)
where p , the fraction of micellized surfactant, was obtained from the self-diffusion measurements described in the previous subsection. 6(free) was measured on a sample with a surfactant concentration below cmc. Data were collected on a Varian XL-300 spectrometer using a digital resolution of 0.125 Hz (corresponding to 0.002 ppm). TMS, in a small capillary tube within the IO-" sample tube, was used as a chemical shift reference.
4. Results and Discussion Since the shift assignments of DAPS are not established, the assignment of the I3C spectra had to be made by correlating the 13C spectra with proton connectivity information in C0SY3O spectra (Figure 1) and heteronuclear 'H-I3C shift correlation spectra" (Figure 2). Note that the actual COSY and HETCOR experiments were made on the octyl analogue of DAPS, for the purpose of providing primarily information of the shifts of the carbons near the nitrogen and the sulfur atoms. Shift assignments along alkyl chains are well established and detailed assignment rules for long aliphatic fatty acid chains have recently been provided by Bengsch et al.32 The final assignment from the combined sources of information is shown in Figure 3. ~
~~
(30) Aue, A. P.; Bartholdi, E.; Emst, R. R.J . Chem. Phys. 1976,64,2229. (31) Bax, A. J . Magn. Reson. 1983, 53, 517. (32) Bengsch, E.; Perly, B.; Delenza, C.; Valero, A. J . Magn. Reson. 1986, 68, 1.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
5282
Jansson et al.
PPM
20 -
30 -
40
-
--
50 -
60 QD 0
70
-
040.1
5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH 3
1 2 3 4 %S-CH2 CH2 CH2 N
I
I
1
I
1
1
1
I
1
I
I
I
I
(CH3)2
a m
5 3
6
1
12118
7
136
2
030
-
020
-
010
9
-
a
@
14
10
@
e
-
-
Figure 3. Assignment of the I3C spectra of DAPS.
4.1. The Evaluated S, ref, and rCsObtainedfor DAPS. Figures 4 and 5 display order parameter and fast correlation time profiles of a 10 wt % solution of DAPS (0.36 M), obtained from the two-step model analysis of I3C relaxation and NOE data. Statistical confidence intervals corresponding to an 80% level were found to be approximatively h0.03 and h5 ps for S and ref, respectively. Since it was not possible to resolve the bands for carbon-9 and -10 at lower magnetic fields, average values of S and rCfare given in these figures. Figure 4 shows that order parameters decrease from the nitrogen atom toward both ends of the surfactant. A comparison of the decaying trends of S along the alkyl chain and the headgroup shows that the order decay is more rapid in the latter case. The relatively high S values observed for the nitrogen-bound methylene carbons (S = 0.30) indicate that the presence of bulky -N(CH3)2- groups, in combination with the anchoring of the surfactant headgroup on the micelle surface, limits the number of possible orientations for methylene C-H bond vectors. Order parameters of the same magnitude have been observed for the a-methylene carbons in DOTAC and CTAC (dodecyl- and cetyltrimethylammonium chloride) micellar systems3 The low S value of the nitrogen-bound methyl group, which is caused by methyl rotations around the nitrogen-carbon bond, is also in agreement with results obtained for these surfactants. However, in contrast to the distinct plateaus in the order parameter profiles which was observed for the cationic surfactants, the decrease in magnitude of the order parameter along the alkyl chain is found
VI
5 f
I
I
I
I
I
I
I
@.
40.00
20.00
I
I
0,s CH2 CH2 C H 2 - i - CH2 C 5 CHI CH2 CH2 Ck$ CH2 CH2
9CH3
I (CH3)2
Figure 5. Fast correlation times, obtained from the two-step model analysis of "C relaxation and NOE data, of 0.36 M DAPS at 25 " C as a function of carbon position in the surfactant chain.
Conformation of (Decyldimethy1ammonio)propanesulfonate b
a
I
!
I I
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5283 TABLE I: Parameters Extracted from the Band-Shape Fitting Procedure at 25 O C
J(o)/J(~o)
R1(I4NI/Hz
1.21 f 0.02"
23.7 3~ 0.3
JC-N/HZ* 4.078 f 0,001
R0("C)/Hz 2.7
0.2
was "The error limits correspond to a 80% confidence interval. obtained by fitting 60 OC data and was locked during the fitting procedure for the 25 OC data.
Figure 6. "C resonance of the nitrogen-bound methyl groups, described by ca. 1000 points at 60 OC (a) and 25 OC (b). The solid line is the theoretical line shapes calculated from the best fit of the experimental points to eq 9 and 10. The amplitude is in arbitrary units.
to be almost continuous. This could be a consequence of shorter alkyl chains and larger S values at the micellar surface in the DAPS system compared to trhe cationic surfactant systems. The fast correlation times, corresponding to complex local motions of C-H bond vectors, are shown in Figure 5. The main features of the correlation time profile have an overall resemblance with data obtained by Ahlnas for the nonionic surfactant ClzE5 (hexaoctaethylene glycol mono-n-dodecyl ether).33 The correlation time profile is considerably different from those obtained for ionic surfactant^.^,^ Ionic surfactants show maximum Tcf-values somewhere between the headgroup and the middle of the hydrocarbon chain, whereas a distinct maximum in T~~ is found at the nitrogen-bound methylene carbons for DAPS. Since a segment in the middle of the alkyl chain requires a high degree of cooperative motion of other molecular segments in order to reorientate, the maximum rcf-value should be located at the middle of the chain for a free surfactant molecule. Therefore, a displacement of the maximum rcf-value toward the surfactant headgroup is a consequence of motional constraints due to anchoring of the headgroup at the micellar surface, which implies that the anchoring is stronger for the zwitterionic surfactant than for ionic surfactants. This is further confirmed by the higher T~~ values observed for DAPS, for which the maximum T~~ is 80 ps as compared to 20-30 ps for ionic surfactants. The high Tc'-value is related to the bulkiness of the zwitterionic headgroup, which restricts the motional freedom of the micellized surfactant headgroup. The slow correlation time, corresponding to micellar reorientation and surfactant lateral diffusion, was evaluated to be 1.8 f 0.8 ns (error limits corresponding to 80% confidence intervals). The large error limits of T~~ imply that I3C data do not determine slow correlation times with the same accuracy as fast correlation times. This is generally found in all I3C based two-step model calculation^.^ A qualitative explanation is that I3C measurements only sample the high-frequency region of the spectral density function where the w dependence of the spectral density function is rather low. The determination of T~~ is therefore inherently more accurate for low-frequency nuclei like I4N or zH.4 4.2. Comparing the Two-step Model Results with a BandShape Analysis. The motional spectral densities J ( w ) and J(2w), where w denotes the Larmor frequency of l4N, were obtained directly from a band-shape analysis of the I3C signal of the nitrogen-bound methyl group. In order to calculate J ( o ) / J ( 2 w ) the values of R 1 ,Ro,and JC-N have to be known or else be fitted to the I3C band shape (see Theory and Methods). It can be anticipated that the quality of the experimental fit is more sensitive to the choice of JC-N at 60 O C (Figure 6a) than at 25 "C (Figure 6b). Therefore, Jc-Nwas determined by fitting all parameters to the methyl band shape at 60 "C, assuming orthogonality of with respect to the other fitted parameters. A JC-N value of 4.018 H z was obtained and was subsequently used in the band-shape analysis at 25 "C, assuming a negligible temperature The result of the band-shape analysis at 25 dependence of JC-N. (33) Ahlnas, T. Ph.D. Thesis, University of Lund Sweden, 1986.
0.00
c
i
I
' '
-OWL " ' ' ' ' ' ' ' ' ' $5- CH2 CH2 CH2-N- CH2 CH2 CH2 CH2 Ct$ CH2 CH2 Cl$ CH2 CH3
1
,C&
Figure 7. Difference in I3C chemical shifts between micellized and nonmicellized DAPS as a function of carbon chain position at 25 OC.
"C is displayed in Table I. The relatively small confidence interval in Table I reflects that the minimum in the error sum corresponding to the extracted parameters is very sharp. The linebroadening component, Ro,was evaluated to be close to the l / T 1 value measured for the methyl carbon (2.6 Hz). The orthogonality of R l and Ro with respect to J ( w ) / J ( 2 w ) was tested by locking J ( w ) / J ( 2 w ) at different values ( 1 . l , 1.3, and 1.4) and fitting Rl and Ro to the band shape. Since no good fits were obtained, the conclusion is that the spectral density ratio is to a large extent orthogonal to the relaxation rates in the band-shape fit. From the values of T$, T ~ and ~ S , extracted from the two-step model and by use of eq 3, spectral densities can be evaluated at the same angular frequence as in the band-shape analysis. The validity of the two-step model can thus be tested.20 Although two-step model calculations on I3C data do not provide information of S and rCffor the nitrogen segment, a relevant comparison can anyhow be made. A numerical trial and error analysis shows that J(w)/J(2w) is more or less determined by the Tcs-value,for relative large variations of S and T~~ (the magnitudes of J ( w ) and J ( 2 w ) are affected by varying S and T ~ however). ~ , If the nitrogen S value is assumed to be somewhere between 0.15 and 0.30 and the rcf-value between 40 and 90 ps (reasonable values when considering order parameters and fast correlation times of neighboring C-H vectors in Figure 4 and 5 ) a T~~ value of 1.8 ns corresponds to a J ( w ) / J ( 2 w )value of 1.09 f 0.04. If we also take into account ~ f 0.8) the J ( w ) / J ( 2 w ) the uncertainty in the Tcs-value ( T =~ 1.8 value from the two-step model data is calculated to be 1.14 f 0.1 1. This should be compared to the result of the band-shape analysis, J ( w ) / J ( 2 w ) = 1.21 f 0.03. Thus, the values agree within experimental uncertainty. Applying eq 3, 11, and 12 and using the values from the band-shape analysis of J ( w ) / J ( 2 w ) and R , (1.21 and 23.8 Hz, respectively) together with the measured doublet splitting in the hexagonal phase (9280 Hz), one obtains independent of the values of S and T~~ a slow correlation time of 2.5 ns. It should be noted that this calculation is based on the assumption that S in eq 12, i.e., for the hexagonal phase of DDAPS, is the same as in eq 3, Le., for DAPS micelles. This is justified with the notion that the local molecular properties for surfactants depend only weakly on the geometries of the aggregate they reside in.4 If S of the nitrogen atom is extracted by interpolation from C-H order parameter data in Figure 4 (which results in a value of S = 0.30) T~~ is calculated to be 80 ps. This value is close to the correlation times found for the methylene carbons adjacent to the nitrogen atom from the I3C relaxation and NOE measurements. This gives further ev-
5284
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
lSO 1.LO
Jansson et al.
5 L OoL
e .0e
I
130 120
l.1°
100
1
I
0
1 t
0 0 c
V
0
i
0
b
0 00
1
I
I
0 05
0 10
0 15
o
1
0 20
idence on the relevance of the two-step model description of the molecular motion in this system. 4.3. Carbon Shift Changes upon Micellization. The chemical shift changes on micellization give qualitative information on any associated conformational change. Since a downfield shift is normally related to an increasing population of trans conformers,+* it can be inferred from Figure 7 that surfactant micellization is accompanied by a partial changeover from gauche to trans conformations in the alkyl chain. This effect is more pronounced for the methylene segments in the middle of the alkyl chain than for those at the end of the surfactant chain, in agreement with other st~dies.~,~~ In contrast to the other carbons, the nitrogen-bound methylene carbons show upfield shifts upon micellization, where the largest upfield shift, 0.38pm, was found for carbon-5. If the upfield shift is interpreted in terms of an conformational change, the increased probability of gauche conformations at carbon-5 indicates that the surfactant headgroup is tilted relative to the normal of the micelle surface. It is known from studies of ionic surfactants that changes in chemical shifts for carbons at the micellar surface depend not only on conformational changes, but also on changes in micellar headgroup interactions and hydration:*34 implying that alternative explanations exist. The conformational interpretation is supported by observations made in studies on aggregated zwitterionic surfactants, however. 31PNMR measurement^^^ and neutron diffraction experiment^^^ employing oriented bilayers of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine) data both indicate that the average orientation of the zwitterionic headgroup is parallel to the bilayer surface. 4.4. Self-Diffusion Measurements of DAPS and DAPSIOctanol. The self-diffusion coefficients of DAPS and the micellar solubilized probe, HMDS, of a 10 wt % (0.361 M) solution of DAPS in D 2 0 were found to be 9.8 X IO-" and 6.4 X IO-" m2 SKI, respectively. Since the free surfactant self-diffusion coefficient was determined to be 40.1 X IO-" m2 s-l (determined by selfdiffusion measurements on a solution of 0.8wt % DAPS) two-site model calculations result in a free surfactant concentration of 36 mM. Lindman et al. found that the free surfactant concentration of DDAPS (the dodecyl analogue of DAPS) is more or less constant over a large concentration range,37implying that the free amphiphile concentration, 36 mM, is close to the cmc of DAPS. H e r r m a r ~ nusing ~ ~ static light-scattering measured the cmc of DAPS in H 2 0 to be 39 mM, a value in good agreement with our results. Since intermicellar interactions contribute to the selfdiffusion coefficient of the micelle, it is not possible to directly relate the self-diffusion coefficient to the size of the micelle, ~~
002 I 0 00
0 25
x oc tanol Figure 8. Ratio of micellar self-diffusion coefficients (monitored by solubilized HMDS diffusion) of pure DAPS micelles and DAPS/octanol micelles as a function of molar ratio of octanol at 25 "C.
~~~~~~~
0
003
~~
(34) Ahlnas, T.; Siiderman, 0. Colloids Surf. 1984, 12, 125. (35) Griffin, R. G.; Powers, L.; Pershan, P. S. Btochemisrry 1978, 17, 2718. (36) Worcester, D. L.; Franks, N. P. J. Mol. Biol. 1976, 100,359 (37) Faucomprt, B.; Lindman, B. J . Phys. Chem. 1987, 91, 383. (38) Herrmann, K. W. J . Colloid Inferface Sci. 1966, 22, 352.
I
I
I
0 10
005
i
I
I
025
020
015
Xoctanol
Figure 9. Concentration of nonmicellized DAPS molecules as a function of molar ratio of octanol at 25 "C.
o'40
0.30
r-----l 0
-
0
0
0
0
m
0.20
-
0 0
I
0
0
0
0.10
0
0
0.00
1
$S-CH2
1
CH2
1
1
CH2-f-
1
1
1
1
1
1
1
1
1
1
CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3
(CH3)2
Figure 10. Order parameter, obtained from the two-step model analysis of I3C relaxation and NOE data, in a mixture of 0.36 M DAPS and 0.1 1 M octanol at 25 "C as a function of carbon position in the DAPS sur-
factant chain. however. The magnitude of D(mic) is nevertheless a typical value obtained for small spherical micelles. Lindman et al. concluded from their self-diffusion measurements that DDAPS forms spherical micelles over large concentration ranges.37 Addition of octanol to the DAPS solution has dramatic effects on the micellar association of DAPS; Figures 8 and 9 show how both the size of the micelle and the free surfactant concentration are affected. In Figure 8 the ratio of micellar self-diffusion coefficients obtained for pure DAPS micelles and DAPS/octanol micelles is represented as a function of molar ratio of octanol. An octanol molar ratio of 0.23corresponds to a Do/D value of 1.47, where D" is the self-diffusioncokfficient of the pure DAPS micelle. This would amount to an increase in the micelle volume of about a factor 3 (using the first-order relation V / p= (@/D)3). Figure 9 shows that the free surfactant concentration decreases from 36 mM to 23 mM, which demonstrates a large stabilizing effect of octanol on the micelle as compared to a solution of free surfactants. There have been many studies on effects of alcohol addition to ionic surfactant systems (see, e.g., ref 39-41), and it is well-known that the presence of longer alcohols decreases the cmc of ionic surfactant solutions. The stabilization of the aggregate associated ionic surfactant is attributed to the decrease of the aggregate surface free energy with increasing alcohol c ~ n t e n t . ~ Further ~,~' investigation is needed to determine if this is also the case for the DAPS/octanol system. (39) Rao, I. V.; Riickenstein, E. J. Colloid Interface Sci. 1986, 113, 375. (40) Jonsson, B. Ph.D. Thesis, University of Lund, Sweden, 1981. (41) Almgren, A,; Swarup, S. J . Phys. Chem.1983, 87, 876.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 0.10,
60.00
1
0
1
I
1
1
,
I
I
,
I
l
5285
,
0
0
t
v
0
'E
10 I
.Y O E
co
U
0
P
0
I
0 -0.10
20'w 0 00$S-CH2
CH CH
-k-
O;S-CHZ
CHI CHt-i-
CH2 CH2 CHt CH2 CH2 CH2 CHZ CH2 CHt CH3
CH2 CH2 CH2CH2 CH2 CH2 CH2CH2 CH2 C ,H,
2 1 (CH3)2
Figure 11. Fast correlation time, obtained from the two-step model analysis of I3C relaxation and NOE data, in a mixture of 0.36 M DAPS and 0.1 1 M octanol at 25 OC as a function of carbon position in the DAPS surfactant chain.
(CH3)2
Figure 12. Difference in I3Cchemical shifts between micellized DAPS in DAPS/octanol micelles and pure DAPS micelles as a function of carbon chain position at 25 OC. 010
I
,
I
I
I
,
,
.
A '
'
I
"
'
I
4.5. S, rei, and rcSObtained for the DAPS/Octanol System. The two-step model assumes that one correlation time is sufficient to characterize the slow isotropic motion of the micelle, limiting its applicability to small spherical aggregates. If larger rodlike micellar systems are to be analyzed, the time scale separation has to be generalized to the case of anisotropic overall motion, Le., a three-step or multistep model has to be used.42 For DAPS/ octanol micelles we have an intermediate case; self-diffusion measurements demonstrate that the addition of octanol to a molar ratio of 0.23 increases the volume of the micelle by a factor of three, indicating that the spherical symmetry of pure DAPS micelles is distorted. Despite the micellar growth upon alcohol addition, we have assumed that the two-step model can be used even for the interpretation of DAPS/octanol relaxation data. The micellar growth observed is not so large that two slow correlation times could be distinguished due to the accompanying distortion of spherical geometry. Figures 10 and 11 display the order parameter and the fast correlation time profiles of DAPS in a mixture of 0.36 M DAPS and 0.1 1 M octanol (a molar ratio of 0.23 for octanol). Statistical confidence intervals are the same as for the data in Figures 4 and 5 . Although self-diffusion measurements did demonstrate that octanol has large effects on the surfactant aggregation, a comparison of the order parameter and fast correlation time profiles of DAPS and DAPS/octanol reveals no significant differences between S and rcf of DAPS in the two different systems. Thus, the configurational differences of the DAPS surfactant observed from I3Cshift measurements (see below) are not so large that they exceed the uncertainties of the S value. It also demonstrates that the local motions of the C-H vectors are to a large extent related to intramolecular properties of the surfactant. rCSwas evaluated to be 2.4 f 0.8 ns. Due to the reorientational contribution to T:,' 7: is expected to be larger for the swollen DAPS/octanol micelles compared to pure DAPS micelles, which is in agreement with what was observed. However, the difference (2.4 ns compared to 1.8 ns) is not significant due to the uncertainty of the r: determination. 4.6. Carbon Shft Changes upon Micellar Inclusion of Alcohol. Although the comparison of order parameter profiles does not reveal any conformational changes when adding octanol, the 13C measurements do indicate such effects. In Figure 12 chemical shift differences of micellized DAPS molecules in DAPS/octanol micelles (0.361 M DAPS and a octanol molar ratio of 0.23) as referred to pure DAPS micelles (0.361 M DAPS) are represented as a function of carbon position in the surfactant chain. The tendency of downfield shifts on the alkyl chain carbons upon micellization of the surfactant (Figure 7) is enhanced with the
micellar inclusion of octanol. However, one exception is that the resonance of the methyl carbon is shifted upfield when octanol is added. In order to examine if this was related to the difference in hydrocarbon chain lengths of octanol and the zwittergent, the same experiment was performed when octanol was substituted with hexanol and decanol. In order to make the comparisons between the different alcohols independent of micellar size and shape, hexanol and decanol were added to such amount that the self-diffusion coefficient of the alcohol-containing micelles were found to be the same as for the DAPS/octanol micelles ( D = 1.48D0), corresponding to a molar ratio of 0.28 for hexanol and 0.19 for decanol. The result is displayed in Figure 13, showing that hexanol produces a larger upfield shift for the methyl carbon compared to octanol, whereas decanol addition instead produces a downfield shift. This leads to the conclusion that the upfield shift of the methyl carbon is a consequence of different hydrocarbon chain length of DAPS and octanol. Similar observations have been reported previously. Pentanol addition has been observed to produce downfield shifts for micellized octanoate carbons, expect for the methyl carbone8 It has further been shown that in mixtures of TTAB and CTAB (tetradecyl- and cetyltrimethylammonium bromide) the terminal methyls have different chemical shifts. This was explained in terms of an increased chain folding (increased gauche population) of the longer chain near the apolar end as compared to the shorter alkyl chain.43 An alternative interpretation to this phenomena has been presented by De Weerd et al. They discussed the difference in the chemical shift in terms of changes in micellar chain packing, affecting the van der Waals interactions of the chains and therefore also the
(42) Ntry, H.; Soderman, 0.;Canet, D.; Walderhaug, H.; Lindman, B. J . Phys. Chem. 1986, 90,5802.
(43) Ulmius, J.; Lindman, B.; Lindblom, G.; Drakenberg, T., J . Colloid Interface Sei. 1978, 88, 6 5 .
0 05
E
W I
0.00
0 05
$5- CH2 CHt CH2-f-
CH2 CH2 CH2 CHt CHt CH2 C? CHZ CH2 CH3
(CH3)2
Figure 13. Difference in I3C chemical shifts between micellized DAPS in DAPS/hexanol micelles and pure DAPS micelles (0)and between micellized DAPS in DAPS/decanol micelles and pure micelles (A) as a function of carbon chain position at 25 OC.
_
_
_
~
_____
J . Phys. Chem. 1987, 91, 5286-5291
5286
shielding of the carbon nuclei.44 Figure 12 shows that the resonances of the methylene carbons adjacent to the nitrogen atom are shifted downfield when octanol is added. If the observation is interpreted in terms of surfactant headgroup tilting, octanol decreases the angle of the headgroup with reference to the normal of the micelle. The O H group of octanol is probably located close to the positively charged nitrogen atoom on the micelle surface, which may affect the headgroup orientation. However, since the magnitude of the chemical shift change is rather small, other sources, such as contributions from changes in intermolecular interactions or solvent interactions, may be of importance. Figures 12 and 13 show that the effect on the chemical shifts of the surfactant headgroup is the same, independent of alcohol. Differences in magnitudes of the chemical shift changes on the headgroup can be related to the different alcohol molar ratios used. 5. Conclusions The interpretation of I3C relaxation and NOE data leads to (44) De Weerd, R.; De Haan, J.; Van den Ven, L.; Buck, H. J . Phys. Chem. 1982, 86, 2528.
the conclusion that the zwitterionic headgroup DAPS is more firmly anchored at the micellar surface as compared to headgroups of ionic surfactants, and that its local motions are also more restricted. Furthermore, the changes in I3C chemical shifts upon micellization indicate that the headgroup is tilted relative to the normal of the micellar surface. Addition of octanol has a large stabilizing effect of the micelle, decreasing the free surfactant concentration and increasing the micellar aggregation number. The influence of octanol on the order parameter and fast correlation time profiles is minor, demonstrating that the local molecular properties depend only weakly on the environment. However, I3C chemical shift measurements demonstrate that octanol does induce conformational changes of micellized DAPS.
Acknowledgment. We thank Professor M. Almgren and Dr. 0. Soderman for helpful discussions. Financial support from the Swedish Natural Sciences Research Council is gratefully acknowledged. The Knut and Alice Wallenberg foundation is thanked for a grant that financed the purchase of the XL-300 spectrometer. Registry No. DAPS, 15163-36-7; octanol, 11 1-87-5.
Photophysical Behavior in Spread Monolayers. Dansyl Fluorescence as a Probe for Polarity at the Air-Water Interface F. Grieser, P. Thistlethwaite,* R. Urquhart, Department of Physical Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia
and L. K. Patterson Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: February 17, 1987)
The emission spectrum of N - [ 5-(dimethylamino)naphthalene-I-sulfonyl]dihexadecylamine(dansyldihexadecylamine) in monolayers at the air-water interface has been studied. In some cases sudden shifts in the dansyl emission can be correlated with particular features of the surface pressure-area isotherms. These spectral shifts can be explained in terms of a change in the conformation of the head group on the surface and with aggregation of the dansyldihexadecylamine. In other cases the dansyl emission shows a blue shift with increasing compression that can be associated with reduced head-group hydration.
Introduction Over the past 10 years there has been widespread use of fluorescent probes aimed at characterizing the interfaces of micelles, vesicles, and membranes, as well as active sites in biological macromolecules.’“ Shifts in the pK, of acid-base indicators away from the value in bulk solution have been used to determine the surface potentials of micellar ~ y s t e m s . ~In , ~such studies it has been customary to assign part of the pK, shift to “nonelectrostatic” factors. This nonelectrostatic effect has often been associated, in a somewhat vague way, with “the difference in dielectric properties” between water and the interfacial region. While there are many reasons to think that the dielectric constant itself is not a particularly appropriate parameter in this context, there has been general agreement in assigning to the interfacial region a (1) Law, K. Y . Phorochem. Photobiol. 1981, 33, 799. (2) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977,81, 2176. (3) Azzi, A. Q.Rev. Biophys. 1975, 8, 237. (4) Loew, L. M.; Cohen, L. B.; Salzberg, B. M.; Obaid, A. L.; Bezanilla, F . Biophys. J . 1985, 47, 71. (5) Walsh Kinnally, K.; Tedeschi, H.; Maloff, B. L. Biochemistry 1978, 17, 3419. (6) Dederen, J. C.; Covsemans, L.; De Schryver, F. C.; Van Dormael, A. Photochem. Photobiol. 1979, 30, 443. (7) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Fennel1 Evans, D. J . Phys. Chem. 1985, 89, 2103. (8) Fernandez, M. S.; Fromherz, P. J . Phys. Chem. 1977, 81, 1755.
0022-36S4/87/2091-S286.%01 . S O / O
“polarity” lower than that of bulk Studies with polarity-sensitive probes have tended to support the conclusions of the pK, studies.’s2 It is not clear whether this reduced polarity is entirely explicable on the basis of the reduced concentration of water in an interface consisting of water and head group^,^ or whether there is an additional contribution associated with water structuring, due to the proximity of the hydrocarbon tail group^.^.'^ Alteration to the H-bond structure of the interfacial water by H-bonding head groups might also be significant. The polarity probe approach can in principle be applied to air-water monolayer films. The use of spread monolayers is of particular interest due to the extra degree of freedom provided by the ability to continually compress the monolayer and alter in a controlled way the interactions among components of the layer. Furthermore, this model system may, by means of force-area isotherms, be monitored for changes in molecular organization and even phase transitions. Recent work has demonstrated the feasibility of making steady-state fluorescence spectral measurements as well as time-resolved fluorescence measurements on air-water mono(9) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 1 , p 241. (10) Ramachandran, C.; Pyter, R. A,; Mukerjee, P. J . Phys. Chem. 1982, 86, 3198.
0 1987 American Chemical Society
